1 Motti: This battle station is the ultimate power in the Galaxy! 120km in diameter, with an inhabitable shell 2km thick...2 Motti: We have space for 700,000 troops, not to mention countless bottomless chasms spanned by narrow walkways with no guard rails.3 Stormtrooper: Lord Vader, we've found the rebel base!3 Vader: Where?3 Stormtrooper: It was here inside the Death Star all along!4 Stormtrooper: We weren't patrolling that 875 billion cubic metres. I mean, it's only 1% of the habitable volume. We just missed it.4 Vader: You were saying?

Thanks to Steven Ehrbar for pointing out the implications of the arithmetic behind the Death Star.
And yes, those dimensions are canonical.

2012-03-05 Rerun commentary: The surface area of the Death Star is a touch over 45,000 square kilometres. That's roughly the size of Estonia, or Denmark. Not so big, you might say. But imagine the entire surface covered with human habitation to the height of 2 kilometres. That's well over twice as tall as the Burj Khalifa. Packed solid over the entire area.